gather-topology: also save /proc/self/cgroup

/proc/self/cpuset isn't enough in some cases.

Thanks to Benedikt Morbach for the patch.

This commit fixes #111.

(cherry picked from commit b9388df)

tests/linux/hwloc-gather-topology.in

@@ -91,6 +91,7 @@ cat << EOF | while read -r path ; do savefile "$destdir/$basename" "$path" ; don
 /proc/mounts
 /proc/stat
 /proc/self/cpuset
+/proc/self/cgroup
 EOF
 
 # Get all files from the given directory